Combustion engine driven industrial trucks require a transmission in order to provide the desired torque at the desired speed. It is desirable that the industrial truck can be driven forwards and backwards at the same high maximum speed.
In motor vehicles, it is known to use a clutch and a hydrokinetic converter in combination with single-step or multi-step transmissions. However, such a system has only limited suitability for industrial trucks, as at low driving speeds it has low efficiency. Furthermore, a clutch function is required for the independent operation of the lifting function of the industrial truck.
The disadvantages of a hydrodynamic transmission system do not occur with a hydrostatic drive train. It uses a continuously variable conversion range and allows easy reversing. However, the poor efficiency at faster driving speeds opposes the relatively high efficiency of driving at slower speeds. As a result of the multiple hydraulic-mechanical conversion of the total driving power, the efficiency of a mechanical gear changing mechanism is not achieved. A further disadvantage is the relatively high construction cost, due to the high hydraulic pressures up to 400 bar and the susceptibility of the system to defects, in particular with regard to leakage.
Finally, it is also known in the industrial truck field to carry out the transmission of forces by means of an electric transmission. In an electric transmission, all the energy generated by the combustion engine is converted into electrical energy by means of a generator. The electrical energy is reconverted into mechanical energy by means of an electric motor. Within predefined limits, drive controls for electric machines allow any gear ratios between the generator and electric motor. Hence, such a system has the advantage of particularly good controllability. The low efficiencies caused by the high losses in the control mechanisms and electric machines are, however, disadvantageous.
Existing driving solutions in combustion engine operated industrial trucks generally involve considerable energy losses in certain operating situations. Furthermore, they are not capable of returning energy to an accumulator when braking or lowering loads. Emissionless operation is also impossible which almost excludes the use of such industrial trucks in enclosed spaces. A further disadvantage of the former driving systems lies in the fact that the components have to be designed for peak loads whilst the mean loading of the components is clearly below such values. Thus a higher construction cost is incurred which is not required for the mean value.
A continuously variable, hydrostatic-mechanical power shift transmission for industrial trucks is known from DE 138 15 780 which is intended to achieve continuously variable hydrostatic starting or reversing with reduced construction cost. The known transmission has a plurality of clutches and gearwheel switch stages. Emissionless operation of the driving system is not possible and the kinetic energy when braking and lowering loads cannot be recovered. Purely hydrostatic starting leads to poor transmission efficiency until switching to power branching transmission. Furthermore, reversing at maximum speed is not possible.
A hydrostatic-mechanical drive for industrial trucks is known from DE 197 47 459 which is intended to utilise the advantages of a hydrostatic drive combined with a mechanical drive, in that the total efficiency of the transmission is increased via the mechanical branch. Such a driving system can be designed such that it can be driven from a standstill to maximum speed, in a preferred direction of travel, with good transmission efficiency. In the other direction of travel the efficiency is low in the maximum speed range. Emissionless operation of the driving system is not possible and it is also not possible to recover braking- and lowering load energy.
A two-branch gear transmission and an operating method is known from DE 198 03 160 which is intended to improve the efficiency behaviour and the service life of the driving system. In addition, a rotary brake which is controllable depending on the driving state of the vehicle, is intended to stop a shaft temporarily or permanently when a shaft exceeds a maximum speed. Emissionless operation of the driving system, which is not designed for industrial trucks, is not possible and it is also not possible to recover braking- or load energy. Furthermore, reversing at maximum speed also cannot be achieved.
Combustion engine driven driving systems for industrial trucks are known from DE 199 55 311, DE 199 55 312 and DE 199 55 313, in which a combustion engine and at least one electric machine generate the drive energy. The known driving systems contain at least one collecting transmission for combining the energy of the combustion engine and the electric machine. It is therefore intended that the performance is improved as a whole, with continuously variable good controllability and an increase in total efficiency. The known driving systems require mechanical brakes in the driving state, in order to achieve the required minimum braking even at maximum speed and without oversized electric machines. The combustion engines in the known driving systems are rigidly connected to the subordinate transmission system so that during emissionless electrical operation, the combustion engine idles and is subject to loss.
The object of the invention is to produce a combustion engine driven, two-branched driving system for industrial trucks which can be operated without emissions, allows recovery of braking- and lowering load energy and, in particular, can dispense with mechanical brakes in the driving state.